1. Field of the Invention
The field of the invention relates to the manufacture of float glass and more particularly to a method for preventing defects resulting from metal dripping from the ceiling of the forming chamber above the metal bath within the forming chamber onto the glass ribbon passing through the chamber.
2. Prior Art
The manufacture of flat glass by the float process involves delivering glass at a controlled rate onto a relatively wide bath of molten metal (normally tin) and advancing it along the bath surface of molten metal under conditions which assure that a sheet of glass of a desired thickness and optical property may be withdrawn at the exit end of the forming chamber. Above the bath of molten metal, the forming chamber holding the molten metal has a headspace containing a reducing to inert atmosphere, a refractory ceiling extends over the headspace and above the ceiling a plenum or headspace for introduction of a forming or nitrogen gas which enters the headspace through the ceiling and prevents deterioration of the metal bath. The plenum also contains the electrical connections for the candle heating elements utilized in the bath.
The forming gas is normally nitrogen with some small percentage of hydrogen. The purpose of the forming gas is to prevent the oxidation of the tin. The forming chamber is tightly closed and a positive pressure of the forming or nitrogen gas is maintained in the forming chamber to prevent the ingress of air containing oxygen. While the conventional practice has resulted in production of fine quality glass there remains a continuing difficulty with a defect known as "tin drip" which results from droplets of molten tin which are on the upper surface of the glass when it is removed from the forming chamber. These tin drips are the result of dripping from the ceiling of the forming chamber. The occurrence of tin drip is a chronic problem in the operation of the forming chamber, but is of particular difficulty when a temperature change is required in the forming chamber. The heating of the roof candles results in affecting the viscosity, surface tension and chemical reactivity of tin droplets on and near the candles leading to their agglomeration and falling from the roof onto the glass. It would be desirable to minimize the formation of elemental tin droplets on the candle and roof structure of the forming chamber.
In U.S. Pat. No. 4,019,855 to Snow it is suggested that the problem of tin drip be reduced by utilization of a halogen or halide fluxing agent which when utilized at infrequent intervals would cause the molten metal deposits on the roof to coalesce and drop onto the glass ribbon or molten metal bath at controlled occasions.
In U.S. Pat. No. 3,597,178 to Tilton and in U.S. Pat. No. 3,494,755 to Montgomery, it is suggested that the tin be condensed in a controlled manner onto a structure within the forming chamber in order to control it in a manner in which it does not drip onto the glass ribbon but rather onto the tin. In Montgomery a curved support overhangs the glass ribbon in such a manner that the tin droplets drip off the edge of the support in an area beyond that covered by the glass ribbon. In Tilton a wire mesh material is passed through the forming chamber where it collects volatiles for removal to an area outside the bath where the screen is cleaned.
It is also known to use lances of ambient temperature forming or nitrogen gas to try and blow tin and other deposits from the roof of the forming chamber. In U.S. Pat. No. 3,811,854 it is disclosed that stones in the furnace refiner roof may be removed by blowing gas at the roof deposits.
There remains a need for a method of minimizing roof contamination and tin drip in the forming chamber without the addition of expensive and complicated apparatus to the chamber and without introducing gases or other chemicals which bring a new material into the forming chamber environment that may contaminate the glass or shorten the life of refractories within the bath. Further, it is important than any material utilized to reduce tin drip does not contaminate the molten metal bath or the glass. A control method not involving process interruptions is also desirable.